Low voltage light bulbs typically comprise one or more incandescent elements in a glass envelope. At best, such incandescent bulbs have short lifetimes. In addition, such incandescent light bulbs are fragile, and if dropped have even shorter lifetimes. In addition, these incandescent light bulbs are inefficient at converting electric energy into visible light, i.e. photons. The brightness of an incandescent light bulb is generally ay function of the voltage applied. In flashlight applications, to get a brighter light one needs to use more batteries. However, a different low voltage incandescent light bulb is required for each discrete number of battery cells.
Alkaline batteries typically provide a voltage of about 1.5 volts per cell. An incandescent bulb that is designed to be powered by one cell will burn out if powered by two or more cells in series. On the other hand, a bulb designed to operate using 4.5 volts, provided for example from three alkaline cells in series, will not produce much light if powered by a single cell. When powered by two cells such a device will produce a light having a yellowish cast due to the lower temperature filament. When powered by a single cell the light emitted from such a device will be very dim. Therefore, in order to provide sufficient light output, a different light bulb is needed for each combination of battery cells.
The required multiplicity of light bulbs is further compounded with use of rechargeable batteries. Nickel Cadmium cells (NICAD, for example, typically have a voltage of about 1.2 volts per cell. A bulb designed for use with three alkaline cells, however, will not provide sufficient light if powered by three NICAD cells. Thus, a different light bulb is required for each combination of NICAD cells. Needless to say, a single bulb using incandescent technology that can be usefully operated over a large input voltage range would be highly desirable. Applicant's invention comprises such a light bulb.
It is known in the art that light emitting diodes, i.e. LEDs, can overcome some of the limitations inherent with incandescent light bulbs. However, the applied voltage must be high enough to overcome the characteristic voltage drop of the LED. Typically, a preferred method to operate an LED is to use a voltage higher than the turn on voltage of the LED, and to limit the current through the LED with a current limiting resistor. This requires using a voltage higher than that actually required by the LED. Such a method, however, prevents LEDs from being used as lighting elements with very low voltage systems. In addition to voltage-related problems, light emitting diodes can be destroyed by driving too much current through the device. Thus, use of an LED requires adjustment of both the voltage and current supplied to that LED.
Prior art LED light bulbs are designed for use with only one specific voltage. This specified voltage must necessarily exceed the voltage drop of the LED. In addition, these prior art devices include one or more LEDs in combination with one or more dropping resistor(s) to limit the current to the LED(s). Typically such prior art LED light bulbs require three battery cells in series to provide more than four volts to light a white LED.
The difficulties inherent with use of such prior art LED light bulbs are also compounded if rechargeable batteries are used. As noted above; Nickel Cadmium cells (NICAD) typically have a voltage of about 1.2 volts per cell. Using three such NICAD cells only provides about 3.6 volts, which is marginal for some white LEDs. Use of four cells, however, can result in premature LED device failure. Therefore, use of NICAD cells to power an LED light bulb requires four NICAD cells in combination with one or more current limiting resistors. Such a combination is necessarily designed for a specific voltage based upon the voltage drop of the LED and the current limiting resistor(s).
Thus, use of such prior art LED light bulbs is subject to constraints almost identical to use of incandescent bulbs. What is needed is an LED light bulb that can be used over a wide range of input voltages. Such a device can be used interchangeably with, for example, a flashlight using one, two, or three, batteries, where those batteries may be of the non-rechargeable or rechargeable type. Applicant's invention comprises such an LED light bulb.